1. Field of the Invention
The present invention relates to an optical scanning unit used for an image forming apparatus such as a copying machine and a printer of an electrophotographic type, and an image forming apparatus provided with the same.
2. Description of the Related Art
In a laser scanning unit used for an image forming apparatus of an electrophotographic type, laser light emitted from a semiconductor laser is deflected by a rotary polygon mirror and a photosensitive member (surface to be scanned) is scanned with the laser light, whereby an electrostatic latent image is formed on the surface of the photosensitive member.
In most of such laser scanning units, an optical path is bent by a mirror for reflecting laser light to reduce the size of the unit.
In the above-mentioned laser scanning unit, some deviations are caused by the faulty dimensional accuracy and faulty mounting accuracy of constituent components. Among of them, typical deviations are a deviation in parallelism between the generating line of the photosensitive member and the main scanning line of the laser light (inclination of main scanning), a deviation in magnification in the direction of main scanning (magnification of main scanning), a deviation in the irradiation position in the direction of sub-scanning (irradiation position of sub-scanning), and the like.
Conventionally, these deviations are corrected by adjusting the mounting angle and position of a reflecting mirror for reflecting the optical path of laser light.
FIG. 7 shows a mirror construction of the above-mentioned conventional type and a construction for supporting a reflecting mirror of the type and a method for adjusting the mirror will be described below.
A rotary polygon mirror 101 shown in FIG. 7 deflects laser light. The deflected laser light is reflected by a deflecting mirror 102 toward a photosensitive drum 103. Three set screws 106a, 106b, 106c are mounted in a protruding manner on supporting metal plates 104, 105 mounted on a base member (not shown) and the reflecting surface of the reflecting mirror 102 is supported by the tips of the three protruded set screws 106a, 106b, 106c. 
The set screws 106a, 106b support two points in a portion close to one end of the reflecting mirror 102 and the set screw 106c supports one point in a portion close to another end of the reflecting mirror 102. There is provided a construction in which a plane including a triangle formed by the tips of the three set screws 106a, 106b, 106c agrees with the surface of the mirror (reflecting surface) and in which the plane formed by the triangle (=surface of the mirror) can be freely inclined or shifted by screwing or unscrewing the respective set screws or by changing the amount of protrusion of them (refer to FIG. 8 and FIG. 9).
The irradiation position of sub-scanning is adjusted by adjusting the amount of protrusion of the set screw 106a or 106b to incline the reflecting mirror 102 in an A direction. The inclination of main scanning is adjusted by adjusting the amount of protrusion of the set screw 106c or the two set screws 106a, 106b to rotate the reflecting mirror 102 in a B direction. The magnification of main scanning is adjusted by changing the amount of protrusion of the three set screw 106a, 106b, and 106c by the same amount to shift the reflecting mirror 102 in a C direction. The adjustment is made as follows: first, the inclination of main scanning is adjusted; then, the magnification of main scanning is adjusted; and finally, the irradiation position of sub-scanning is adjusted.
Moreover, in addition to the above-mentioned conventional adjustment method, Japanese Unexamined Patent Publication No. 2002-277785 discloses an optical scanning unit that includes a mechanism for supporting the reflecting surface of a mirror at two supporting points and capable of swinging the reflecting surface of the mirror around the two supporting points and has a swinging axis connecting the two supporting points set in such a way as to be in close agreement with the path of scanning light moving on the mirror.
Meanwhile, Japanese Unexamined Patent Publication No. Hei 11-352427 discloses a construction in which supporting members provided on both ends of the reflecting mirror can move together.
Moreover, Japanese Unexamined Patent Publication No. 2000-180748 and No. 2002-277785 discloses a construction in which the reflecting surface is made to agree with the rotational center of the mirror.
However, according to the above-mentioned adjusting methods, when the amount of protrusion of the set screw 106a is changed so as to adjust the irradiation position, the surface of the reflecting mirror is rotated around an imaginary straight line passing the tips of the other two set screws 106b, 106c and hence the inclination and magnification of main scanning are deviated at the same time. Then, the inclination and magnification of main scanning are readjusted, which in turn deviates also the irradiation position of sub-scanning. Therefore, these three adjustments need to be repeated several times to converge the respective adjustment items on specifications. This presents the problem of increasing the number of man-hours required for adjustment work.
Moreover, according to the construction disclosed in Japanese Unexamined Patent Publication No. 2002-277785, by causing the rotational axis of the mirror to agree with the scanning line of laser in the longitudinal direction of the mirror so as to adjust the irradiation position of sub-scanning, it is possible to prevent an optical path length from being deviated by the rotation of the mirror. However, according to the construction disclosed in Japanese Unexamined Patent Publication No. 2002-277785, the mirror cannot be moved in the direction of irradiation of laser and hence the direction of main scanning cannot be adjusted.